The present invention relates to getter systems comprising one or more deposits of getter material, wherein at least one of the deposits is in contact with a layer of a material having H2O transport properties.
The getter materials and systems are widely used in industry in all applications where it is necessary to maintain a vacuum or to control the composition of the gaseous atmosphere through the absorption of traces of undesired gases.
Getter materials widely used in industrial production are some metals such as titanium, zirconium, niobium, vanadium or hafnium or alloys thereof (and in particular, the zirconium- and titanium-based alloys), which are useful for the absorption of small molecules such as hydrogen, oxygen, water, carbon oxides, and in some cases nitrogen. However, these materials have the limitation of requiring relatively high temperatures (generally higher than 300° C.) for the activation of the getter function, which makes them unsuitable for the use in some devices, for instance those including organic materials.
Examples of applications where it is not possible to resort to thermal activation are panels for thermal insulation filled with polymer foams, as described for instance in U.S. Pat. No. 4,444,821; U.S. Pat. Nos. 5,505,810 and 5,885,682; or OLED screens, described for instance in U.S. Pat. No. 5,882,761.
Another particularly interesting application for getter materials is in microelectromechanical systems, better known in the field by the acronym MEMS, with particular reference to those MEMS comprising an interface with the external environment made of a transparent element; by way of example, the DMDs (from Digital micro Mirror Device) are mentioned.
Among the getter materials which require thermal activation only at relatively low temperatures (compatible with the electroluminescent organic materials which form the active element of the screens using multilayers of electroluminescent organic materials, known in the field as OLEDs), some porous materials, such as active carbons can be mentioned, useful particularly for the absorption of organic substances, or zeolites, silica or alumina, which are useful for the absorption of small-size gaseous molecules. Materials not requiring thermal activation are the anhydrous chemical desiccants, specific for the absorption of moisture, such as the oxides of the alkaline-earth metals, or some hygroscopic salts, such as chlorides (e.g. calcium chloride, CaCl2), perchlorates (e.g. magnesium perchlorate, Mg(ClO4)2), or sulfates (e.g. calcium sulfate, CaSO4).
Because of the importance of this application, in order to exemplify the uses of the getter systems of the invention, reference will be made in particular to the use in OLEDs, but the getter systems of the invention are for general utilization and can be used also in the applications where metals and metal alloys mentioned above are normally used.
The organic multilayer elements arranged inside OLED screens are very sensitive to the presence of gas traces, in particular to humidity which can give rise to two different types of degradation phenomena:
reduction of the screen life due to an attenuation of brightness with time, this phenomenon being associated with the amount of gaseous impurities responsible for the degradation which are present in the proximity of the organic materials multilayer. This type of phenomenon is caused by a concentration of such gaseous impurities being capable of triggering irreversible phenomena of degradation of the organic materials; and
tendency to a spatial non-homogeneity of brightness, this phenomenon being connected to non-uniformity of the concentration of the impurities, with particular emphasis on non-uniformity in the distribution of the concentration of H2O permeating mainly through the adhesive that is used for sealing the OLED cavity. This phenomenon is particularly insidious, as it can appear in relatively short times, and the only way for avoiding the onset thereof is to guarantee a H2O concentration which is as uniform as possible inside the cavity.
A technological solution capable of solving the problems related to the presence of gaseous impurities inside OLED screens thus must guarantee, in correspondence to the electroluminescent organic materials multilayer, low levels of H2O and a concentration thereof being as uniform as possible.
A satisfactory technical solution for OLED screens has not been found yet. For instance, in U.S. Pat. No. 6,833,668 B1 there is described the use of a resin, containing a getter material, being used for sealing the OLED cavity. However, this solution is not able to guarantee uniformity of the H2O concentration.
Another approach is described in the International patent application publication WO2005/050736, which discloses means to adhere a getter composition to an inner surface of electronic devices, where the getter is in particle form dispersed in a suitable binder and a liquid medium.
A different solution is shown in Japanese patent application publication JP 2004-186043, where a distributed deposit of getter material is used along the whole peripheral edge of the active surface of the screen, thus creating a sort of frame of getter material acting as a barrier against the entry of impurities. Also in this case, it is not possible to guarantee a uniform H2O concentration in correspondence to the organic multilayer. Such a concentration is in fact inevitably higher at the center of the device compared to the edge.
Still another known solution is the one described in the U.S. patent application publication 2004/0201347 A1, whose most general embodiment is schematically shown in FIG. 1. The OLED screen 10 consists of a lower substrate 11, an electroluminescent active multilayer 12 being formed on a surface of the substrate 11, and a transparent front panel 13 being coupled by means of spacers 15, 15′ to the lower substrate. Lower substrate 11, front panel 13 and spacers 15, 15′ define an inner cavity 14. The front panel 13 has, on its internal surface, a coating made with an H2O absorber 16, in order to remove the impurities which succeed in diffusing within the internal cavity 14. The absorber 16 is transparent as it must be able to transmit to the outside the light radiation produced by the electroluminescent organic multilayer 12 through the front panel 13.
The electroluminescent organic multilayer 12, in order not to introduce useless complexity into the drawing, is exemplified with a simple rectangle, even though it consists of an assembly of elements, among which are a first series of electrodes, an organic multilayer and a second series of electrodes, which are sequentially stacked. This technical solution is potentially capable of solving the aforementioned problems associated with the permeation of H2O and O2, having an absorber of impurities arranged in the proximity of the organic multilayer, and the layer of absorbing material having a larger extension than the deposit of organic multilayer 12. The main problem with this previously shown technical solution is that, by reaction with the gas to be absorbed, the getter material generally undergoes structural and morphological modifications, for instance swellings, which particularly in the case of the desiccants can be remarkable. Further, as a consequence of the gas absorption, the getter material or the whole system containing the same can undergo other undesired modifications, such as the loss of transparency.